1. Technical Field of the Invention
This invention relates to recovering precious metal and/or metal values from ores including refractory ores, ore concentrates, or ore tailing which include arsenic-, carbon- and/or sulfur-containing components and ores which are refractory to the recovery of precious metal values. Another aspect of the invention relates to a process of roasting refractory precious metal ores wherein added fuels, including sulfur, pyrite or mixtures thereof, are either pre-mixed with the ores or directly fed in during the roasting process. This aspect of the invention includes the proper employment of these added fuels in combination with the above process of recovering precious metals and/or metal values from ores.
2. Background Art
Precious metals, such as gold, occur naturally in ores in different forms. Unfortunately, precious metal ores also frequently contain other materials which interfere with the recovery of these precious metal values, rendering these ores refractory to precious metal recovery. Furthermore, the precious metal content may be at a relatively low level. This low level content compounds the effect of the refractory nature of these ores.
The following patents, hereby incorporated by reference, are illustrative of attempts to deal with refractory components in precious metals and other metals recovery as well as efforts in distinctly different fields addressed to solving the arsenic contamination problems encountered when roasting precious metal and other metal ores having arsenic as an unwanted component present in the ore.
U.S. Pat. No. 360,904 to Elizabeth B. Parnell relates to roasting gold or silver bearing ores using a double roasting schedule with the first roasting at 1100 to 1300 degrees Fahrenheit and the second roasting to 1200.degree. F. to 1600.degree. F. (the time occupied in the second roasting can be reduced by supplying oxygen along with the air).
U.S. Pat. No. 921,645 to J. E. Greenwalt discloses the roasting of ore by heating the ore on a porous granular bed through which air is forced from below.
U.S. Pat. No. 1,075,011 to N. C. Christensen, Jr. discloses a process for treating ore by means of a roasting oven which, by regulation of the fuel supply, may be either oxidizing, reducing, or neutral.
U.S. Pat. No. 2,056,564 to Bernart M. Carter discloses suspension roasting of finely divided sulfide ores. Roasting is in air or oxygen in which the temperature of the mixture entering the roasting chamber is controlled and to a corresponding degree the temperatures within the roasting chamber are thus controlled in an effort to prevent the formation of accretions on the walls of the apparatus.
U.S. Pat. No. 2,209,331 to Ture Robert Haglund discloses a process for the production of sulfur from the roasting of sulfide material in oxygen or air enriched with oxygen so that as soon as the free oxygen has been consumed in the formation of SO.sub.2, the iron sulfide reacts with the sulfur dioxide forming free sulfur and iron oxides.
U.S. Pat. No. 2,536,952 to Kenneth D. McCean relates to roasting mineral sulfides in gaseous suspension.
U.S. Pat. No. 2,596,580 to James B. McKay et al. and U.S. Pat. No. 2,650,169 to Donald T. Tarr, Jr. et al., relates to roasting gold-bearing ores which contain commercially significant amounts of gold in association with the mineral arsenopyrite. The patent describes the importance of closely regulating the availability of oxygen in order to provide enough oxygen so that volatile compounds of arsenic are formed while the formation of nonvolatile arsenic compounds is minimized.
U.S. Pat. No. 2,867,529 to Frank A. Forward relates to treatment of refractory ores and concentrates which contain at least one precious metal, sulfur and at least one arsenic, antimony or lead compound by roasting in a non-oxidizing atmosphere at a temperature above 900 degrees Fahrenheit, but less than the fusion temperature of the material being roasted.
U.S. Pat. No. 2,927,017 to Orrin F. Marvin relates to a method for refining metals, including precious metals, from complex ores which contain two or more metal values in chemical union or in such physical union as to prevent normal mechanical separation of the values. The method uses multiple roasting steps.
U.S. Pat. No. 2,993,778 to Adolf Johannsen et al. relates to roasting a sulfur mineral with its objects being--the production of sulfur dioxide, increasing the completeness of roasting and the production of metal oxides.
U.S. Pat. No. 3,172,755 to Angel Vian-Ortuno et al. relates to a process for treating pyrite ores bearing arsenic by subjecting the arsenic-containing pyrite ore to partial oxidation so as to oxidize only the labile sulfur of the arsenic-containing pyrite and subsequently heating the pyrite ore in a non-oxidizing gas to separate the arsenic from the ore and to form a residual ore free of arsenic.
U.S. Pat. No. 4,731,114 Gopalan Ramadorai et al. relates to a process for the recovery of precious metals from low-grade carbonaceous sulfide ores using partial roasting of the ores following by aqueous oxidation in an autoclave.
U.S. Pat. No. 4,919,715 relates to the use of pure oxygen in roasting of refractory gold-bearing ores at temperatures between about 1000.degree. F. (537.8.degree. C.) and about 1200.degree. F. (648.9.degree. C.). This patent fails to address the problem of arsenic volatilization, is silent on the arsenic content in the ore, and does not address in that context the optimizing of gold recovery from refractory sulfitic, carbonaceous ores or separation of cyanide consuming components before recovery of gold from the ore. The disclosed method requires two fluid beds and stage-wise roasting in these beds and the use of substantially pure oxygen (substantially pure oxygen being defined as at least about 80% by weight.)
European Patent Specification 0 128 887 discloses roasting sulfide concentrates having an average particle size below 1 mm and containing copper and noble metals as valuable metals as well as arsenic as an impurity. Volatization of arsenic is in a circulating fluidized bed under an oxygen partial pressure of 10.sup.-14 to 10.sup.-16 bars and at low temperatures, i.e. temperatures which exceed the breakdown and decomposition temperatures of arsenic compounds. A major part of the solids is removed under the same conditions in a hot cyclone from the suspension discharged from the fluidized bed reactor and is recycled to the fluidized bed reactor. Additional solids are removed from the gas in a second cyclone. After an optional fine purification in an electrostatic precipitator the exhaust gas is discharged through a chimney. The calcine from the circulating fluidized bed and eventually solids collected in the second cyclone are fed to a classical fluidized bed, in which the sulfur containing materials which are present are roasted at an increased oxygen potential. In the event the temperature falls below the sublimation temperature of the arsenic oxides contained in the exhaust gas from the circulating fluidized bed, arsenic oxides may be removed together with the residual solids. That exhaust gas may also contain volatilized sulfur.
German Patent Specification 15 83 184 discloses the removal of arsenic from iron ores and calcined pyrites in a process in which the ores are mixed with calcium oxide or calcium carbonate in an amount of 0.5% to 5% as Ca relative to the weight of the ore and are heated in an oxidizing atmosphere to 800.degree. C. to 1000.degree. C. so that the arsenic is concentrated in a fine-grained fraction. This fraction is separated from the coarser fraction and is leached with acids to remove arsenic. In this patent, in the description of the state of the art in the roasting of pyrites, an addition is described of oxides, hydroxides and various salts of alkali metals and alkaline earth metals. From these additives, corresponding water-soluble arsenates may be formed from the arsenic contained in the ore. The effect of these additives in the roasting stage is constrained by the formation of the corresponding sulfates. The sulfates are almost entirely inactive in a reaction for partitioning arsenic. When the above substances are added to calcined pyrites in an oxidizing atmosphere at 500.degree. C. to 900.degree. C., arsenates will be formed, which may be leached with salt solutions or acid solution. These arsenates should not be dumped in open air dumps. Moreover, the leaching results in an arsenic-containing solution, which is nearly impossible to dispose environmentally in an acceptable manner.
For sulfide ores, any arsenic which is present is an undesired accompanying element and must be removed from the calcine and from the roaster gas. This is typically accomplished by a so-called dearsenication roasting. The arsenic content of the material is volatilized in a roasting zone having a low oxygen content and enters the gaseous effluent as arsenic vapor or arsenic oxide vapor and arsenic sulfide vapor. The above mentioned U.S. patent art deals with such roasting. In the gaseous effluent, arsenic and arsenic sulfides are oxidized to form arsenic oxide vapors under a relatively high oxygen partial pressure.
However, a number of problems are encountered. The dustlike solids contained in the roaster gas are removed at a temperature exceeding the sublimation temperature of the arsenic oxides, which are subsequently separated at lower gas temperatures, or the solids and the arsenic oxides are jointly removed at lower gas temperatures. In the first case, contaminated arsenic oxides will be formed. In the second case, the arsenic which has been removed will be recycled in the process scheme. Recycling is together with the other solids which have been separated, particularly if the solids contain valuable metals and for that reason alone must be recirculated, or the removed solids may be dumped only after taking special precautionary measures because of the arsenic content. In the second case there is also a risk that part of the arsenic oxide may undesirably and unpredictably react with metal oxides to form metal arsenates, e.g., with Fe.sub.2 O.sub.3 to form FeAsO.sub.4. The metal arsenates deposit on the ore particle surfaces and clog the pores of the particle.
Particularly in the roasting of gold ores, the formation of FeAsO.sub.4 on the particle surfaces will involve a higher cyanide consumption in the leaching and a lower yield of gold.
German Patent Specification 1,132,942 disclosed a process of roasting iron-containing sulfide ores, particularly pyrites in which the ores are roasted in a single stage fluidized bed roaster with oxygen-containing gases at 800.degree. C. to 900.degree. C. under an oxygen partial pressure not in excess of 2.9.times.10.sup.-8 atm so that the iron content is reacted to form Fe.sub.3 O.sub.4, some sulfur is sublimated and arsenic, arsenic sulfides and arsenic oxides are vaporized. Solids entrained by the roaster exhaust gas are subsequently removed at temperatures exceeding the condensation temperatures of sulfur and arsenic and the roaster gas is after-burned with a supply of air or oxygen so that the oxygen partial pressure is sufficiently increased to ensure a complete combustion of the sulfur in the purified roaster gas. The arsenic oxides produced by the after burning and removed from the gas stream, will be contaminated by residual dust.
German Patent Specification 1,458,744 discloses the roasting of iron sulfides by a process in which the ores are roasted in a single stage fluidized bed roaster with oxygen-containing gases at 700.degree. C. to 1100.degree. C. and under an oxygen partial pressure of about 10.sup.-2 to 10.sup.-15 atm, whereby Fe.sub.2 O.sub.3 is partly formed, the arsenic which is present is substantially volatilized as As.sub.2 O.sub.3 and the sulfur is volatilized as elementary sulfur. After the solids have been removed from the roaster gas, the oxygen partial pressure in the roaster gas is increased by a supply of air and the elementary sulfur and the arsenic compounds are oxidized. In that process too the volatile arsenic oxides are contaminated by residual dust as they are removed from the gas stream.
From German Patent Specification 30 33 635 it is known that arsenic-containing material, particularly non-ferrous metal ores, may be treated and the arsenic may be volatilized in a first stage at temperatures of 627.degree. C. to 927.degree. C. and under oxygen partial pressures of about 10.sup.-16 bars. The solids are roasted under oxidizing conditions in a second stage. The gas from the second stage is fed in part to a gas purifier and in part to the first stage. Sulfur and oxygen are added to the exhaust gas from the second stage and the arsenic contained therein is completely reacted to form arsenic sulfides, which are partly present as fine dust and partly as vapor. In a scrubber the vaporous arsenic sulfides are condensed and removed together with the solid arsenic sulfides. The arsenic sulfides which have been removed from the scrubbing water are dumped. The presence of SO.sub.2 involves a risk of a formation of arsenic oxides, which must not be dumped because of their solubility. Besides, a high consumption of elementary sulfur is involved.
U.S. Pat. No. 3,479,177 to Veronica et al. relates to a process for removing arsenic from arsenic-containing iron minerals. The process involves adding lime or limestone to the iron minerals which are subjected to heating in the presence of air, but not in the presence of excess oxygen, and in the presence of an additive (CaO or CaCO.sub.3).
None of these patents teaches or suggests roasting ores or refractory ores, ore concentrates or ore tailings of the type described herein for recovery of metals such as precious metals in an oxygen-enriched gaseous environment under conditions as described herein in order to minimize and/or eliminate arsenic volatilization, facilitate arsenic conversion to an insoluble, environmentally acceptable form immobilized in a waste product while reducing the effects of carbon- and sulfur-containing components on metal recovery such as precious metal recovery.
Moreover, none of the references deals with the conversion of arsenic to arsenates of environmentally very stable compounds during roasting e.g. a single stage circulating fluid bed roasting of ores. In fact, the opposite is true. The present invention achieves excellent results in a simpler more efficient manner with outstanding metal, e.g. gold recovery with facile arsenic elimination as an environmental problem, while minimizing leaching cyanide consumption and conserving heat given-off in the roasting process.
Furthermore, none of the references teaches or suggests roasting refractory ores in the presence of the added fuels (sulfur, pyrite or mixtures thereof) in an oxidizing atmosphere in a fluidized bed, in particular, either pre-mixing the sulfur, pyrite or mixtures thereof with the ore or directly feeding these fuels prior to or during the roasting. Moreover, none of the references disclose the use of the sulfur, pyrite or mixtures thereof as added fuel for a roasting process in conjunction with at least one additional fuel selected from the group consisting of hydrocarbons containing 1 to 16 carbon atoms, alcohols containing 1 to 6 carbon atoms, and 1 to 2 COH groups, organic ethers containing 2 to 8 carbon atoms and one oxygen atom, CS.sub.2, and H.sub.2 S. Still further, these references fail to disclose the use of these fuels to lower the ignition temperature of the roasting. In addition, these references fail to disclose the use of these fuels to produce an exhaust gas which is free of or has only low contents of CO, H.sub.2, and hydrocarbons.